The present invention relates to a printing system with a head, which forms at least two different dots having different densities per unit area on an object, for recording multi-tone images by the dots formed by the head, as well as to a method of recording such images.
Color printers, in which a plurality of color inks are discharged from a head, are widely used as an output device of a computer that records a multi-color, multi-tone image processed by the computer. Several methods are applicable to print a multi-color, multi-tone image with three color inks, cyan, magenta, and yellow (CMY). One method is a technique adopted in the conventional printers. This technique expresses the tone of a printed image by the density of dots (frequency of appearance of dots per unit area) while fixing the size of dots formed on a sheet of paper by a spout of ink. Another method adjusts the diameter of dots formed on a sheet of paper, in order to vary the density of dots per unit area. Although the advanced processing of the head for forming ink particles has been improving the density of dots formable per predetermined length or the variable range of the dot diameter, the printers have only the limited printing density (resolution) to 300 through 720 dpi and the limited particle diameter to several ten microns. The resolution of printers is significantly lower than the resolution of silver photography, which has reached several thousand dpi on the film.
Dots are sparsely formed in an area of low image density, that is, in an area of low density of dots to be printed. This increases the degree of granularity and makes the dots conspicuous. A printing system and a method utilizing inks of different densities have been proposed to improve the printing quality. The proposed technique utilizes a high-density ink and a low-density ink for a certain color and regulates discharge of these inks, thereby realizing print with an excellent tone expression. By way of example, a method of and an apparatus for recording multi-tone images are disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. 61-108254. The disclosed technique provides a head for forming deep dots and light dots for a specific color and regulates the number and overlap of deep dots and light dots formed in a predetermined dot matrix according to the density information of an input image, so as to record a multi-tone image.
The proposed printing system utilizing inks of different densities, however, does not give any specific idea on the allocation of high-density ink and low-density ink to the input tone signals of an original image. Inks of different densities are simply allocated in the order of densities to the input tone signals of the image (for example, FIG. 9 in JAPANESE PATENT LAYING-OPEN GAZETTE No. 2-215541).
In a printing system for forming at least two different dots having different densities per unit area (for example, dots by at least two inks of different densities), the object of the present invention is to enable the at least two different dots to be appropriately mapped to tone signals of an original image, thereby improving the quality of a resulting recorded image.
The present invention applies the following structures, in order to realize at least part of the above and the other related objects. The present invention is directed to a first printing system with a head, which forms at least two different dots having different densities per unit area on an object, for recording a multi-tone image by a distribution of the dots, the first printing system including:
input means for successively receiving a tone signal of each target pixel included in an image to be printed;
recording density setting means for specifying a recording density to be realized by at least a selected one of a dot having a higher density per unit area, that is, a higher-density dot, and a dot having a lower density per unit area, that is, a lower-density dot, which are both included in the at least two different dots having different densities per unit area, based on the input tone signal;
first dot formation determination means for carrying out a multivaluing operation based on the specified recording density, and determining whether or not the selected one of the higher-density dot and the lower-density dot is to be formed;
second dot formation determination means for making a result of the multivaluing operation by the first dot formation determination means reflect upon a recording density to be realized by the other one of the higher-density dot and the lower-density dot, causing the other one of the higher-density dot and the lower-density dot to be subjected to a multivaluing operation according to the reflected recording density, and determining whether or not the other one of the higher-density dot and the lower-density dot is to be formed; and
head driving means for driving the head based on results of the determination by the first dot formation determination means and the second dot formation determination means, in order to form the at least two different dots having different densities per unit area.
In the first printing system of the present invention, the recording density setting means specifies a recording density to be realized by at least a selected one of a higher-density dot and a lower-density dot, which are both included in the at least two different dots having different densities per unit area. The first dot formation determination means carries out a multivaluing operation based on the specified recording density, and determines whether or not the selected one of the higher-density dot and the lower-density dot is to be formed. The multivaluing operation may be binary coding, ternary coding, or higher-value coding. In case that the at least two different dots having different densities per unit area consist of only two different dots, the first dot formation determination means carries out the determination for either one of the two different dots. In case that the head can form four different dots, however, the first dot formation determination means may carry out the multivaluing operation simultaneously for two or more different dots. The second dot formation determination means makes the result of the multivaluing operation by the first dot formation determination means reflect upon a recording density to be realized by the other one of the higher-density dot and the lower-density dot. The second dot formation determination means then causes the other one of the higher-density dot and the. lower-density dot to be subjected to a multivaluing operation according to the reflected recording density, and determines whether or not the other one of the higher-density dot and the lower-density dot is to be formed. The head driving means drives the head based on the results of the determination by the first dot formation determination means and the second dot formation determination means, in order to form the at least two different dots having different densities per unit area.
This structure enables a density error due to formation of the selected one of the higher-density dot and the lower-density dot to be reduced by formation of the other one of the higher-density dot and the lower-density dot, so that the tone of an original image is reproduced by a combination of the at least two different dots.
This printing system may be carried out by a variety of embodiments. By way of example, the recording density setting means may specify only a recording density to be realized by the selected one of the higher-density dot and the lower-density dot, or alternatively may specify a recording density to be realized by the selected one of the higher-density dot and the lower-density dot as well as a recording density to be realized by the other one of the higher-density dot and the lower density dot, based on the input tone signal. In the latter case, the first printing system further includes recording density correcting means for computing correction data, which is to reflect upon the recording density to be realized by the other one of the higher-density dot and the lower-density dot, based on the result of the multivaluing operation with respect to the selected one of the higher-density dot and the lower-density dot, in order to correct the recording density to be realized by the other one of the higher-density dot and the lower-density dot. The second dot formation determination means determines whether or not the other one of the higher-density dot and the lower-density dot is to be formed, based on the corrected recording density.
In accordance with one preferable application of the first printing system of the present invention, the first dot formation determination means determines whether or not a first dot selected among the at least two different dots having different densities per unit area is to be formed, based on the input tone signal, prior to determination for formation of the other dots. When no formation of the first dot is determined, the second dot formation determination means determines whether or not a second dot having a different density per unit area from that of the first dot is to be formed. In this application, the first printing system further includes error diffusion means, which computes a difference between a printing density corresponding to the input tone signal and a printing density realized by the formed dots as a density error, based on the determination of dot formation by the first dot formation determination means and the second dot formation determination means. The error diffusion means then distributes the density error to peripheral pixels in the vicinity of a current target pixel of dot formation, in order to reflect upon the determination of dot formation with respect to the peripheral pixels by the first dot formation determination means and the second dot formation determination means.
The present invention is also directed to a second printing system with a head, which forms at least two different dots having different densities per unit area on an object, for recording a multi-tone image by a distribution of the dots, the second printing system including:
input means for successively receiving a tone signal of each target pixel included in an image to be printed;
tone value setting means for specifying a first dot tone value, that is, a tone value to be realized by a first dot selected among the at least two different dots having different densities per unit area, based on the input tone signal;
first dot formation determination means for determining whether or not the first dot is to be formed, based on the first dot tone value;
correction signal computing means for computing a correction signal by adding quantization errors distributed from peripheral processed pixels in the vicinity of the target pixel to the input tone signal;
second dot formation determination means for, when the first dot formation determination means determines no formation of the first dot, determining whether or not a second dot having a different density per unit area from that of the first dot is to be formed, based on the correction signal;
head driving means for driving the head based on results of the determination by the first dot formation determination means and the second dot formation determination means, in order to form the at least two different dots having different densities per unit area; and
error diffusion means for computing a quantization error, which is a difference between the correction signal and a tone value realized by the formed dots, as a density error, based on the results of the determination by the first dot formation determination means and the second dot formation determination means, and distributing and diffusing the computed density error to peripheral pixels in the vicinity of the target pixel.
In the second printing system of the present invention, the input means successively receives a tone signal of each target pixel included in an image to be printed. The tone value setting means specifies a first dot tone value, that is, a tone value to be realized by a first dot selected among the at least two different dots having different densities per unit area, based on the input tone signal. The first dot formation determination means determines whether or not the first dot is to be formed, based on the first dot tone value. The correction signal computing means computes a correction signal by adding quantization errors distributed from peripheral processed pixels in the vicinity of the target pixel to the input tone signal. The second dot formation determination means carries out the processing, based on the correction signal. When the first dot formation determination means determines no formation of the first dot, the second dot formation determination means determines whether or not a second dot having a different density per unit area from that of the first dot is to be formed. The head driving means drives the head based on results of the determination by the first dot formation determination means and the second dot formation determination means, in order to form the at least two different dots having different densities per unit area. The error diffusion means computes a quantization error, which is a difference between the correction signal and a tone value realized by the formed dots, as a density error, based on the results of the determination by the first dot formation determination means and the second dot formation determination means, and distributes and diffuses the computed density error to peripheral pixels in the vicinity of the target pixel.
This structure enables a density error due to formation of the first dot to be reduced by formation of the second dot having a different density per unit area from that of the first dot, so that the tone of an original image is reproduced by a combination of the at least two different dots.
In accordance with one preferable application of either one of the first and the second printing systems, the second dot formation determination means includes: local effect computing means for calculating a local effect from the recording density of the selected dot, which is subjected to determination of dot formation by the first dot formation determination means, and a printing density realized by the selected dot; and recording density correcting means for correcting the recording density to be realized by the other dot by taking into account the local effect, so as to affect the determination of dot formation with respect to the other dot. In this structure, the local effect with respect to the selected dot affects the determination of dot formation with respect to the other dot. This structure makes it difficult to form the lower-density dot in a pixel where the higher-density dot has already been formed, thereby reducing the possibility of polarized formation of the at least two different dots having different densities per unit area. This improves the printing quality.
In accordance with one preferable embodiment, the local effect computing means calculates a difference between the recording density of the selected dot and the printing density realized by the selected dot as a local error, and the recording density correcting means adds a product of the local error and a predetermined weight to the recording density to be realized by the other dot, so as to affect the determination of dot formation with respect to the other dot.
The first dot formation determination means may carry out the determination for the dot having a higher density per unit area or alternatively for the dot having a lower density per unit area. The first dot subjected to the determination by the first dot formation determination means may depend upon the dot formation technique (for example, the error diffusion method or the systematic dither method) as well as the properties of an image to be printed. When the first dot formation determination means adopts a dither method, a systematic dither method is preferably carried out with a threshold matrix of discrete dither.
The head may discharge at least two inks of different densities to form the at least two different dots having different densities per unit area. It is preferable that the at least two inks of different densities include a higher-density ink and a lower-density ink and that a dye density of the lower-density ink is approximately one quarter a dye density of the higher-density ink.
When the head discharges a plurality of chromatic color inks to form chromatic dots as well as an achromatic color ink, such as black ink, to form an achromatic dot, one preferable structure of the printing system includes third dot formation determination means for determining whether or not the achromatic dot is to be formed by the achromatic color ink. When the third dot formation determination means determines formation of the achromatic dot by the achromatic color ink, the preferable structure assumes that the first dot formation determination means determines formation of the selected dot and activates the second dot formation determination means and the error diffusion means. This is because the achromatic color is considered to include the components of the chromatic colors in multi-color printing.
In case that the achromatic dot is formed by the achromatic color ink, the degree of effect of the achromatic dot on a density error with respect to the chromatic color inks may be specified for each chromatic color. The density error may be distributed to density errors with respect to the respective chromatic color inks.
The head of such a printing system discharges two inks of different densities for at least either of cyan and magenta, in order to enable color printing.
The present invention is further directed to a third printing system with a head, which forms at least two different dots having different densities per unit area by a chromatic color ink as well as an achromatic dot by an achromatic color ink on an object, for recording a multi-tone image by a distribution of the dots, the third printing system including:
input means for successively receiving a tone signal of each target pixel included in an image to be printed:
density specifying means for specifying a density to be realized by the chromatic color ink and a density to be realized by the achromatic color ink, based on the input tone signal;
achromatic dot formation determination means for carrying out a multivaluing operation for the achromatic color ink, based on the density to be realized by the achromatic color ink, and determining whether or not the achromatic dot is to be formed by the achromatic color ink;
density correcting means for obtaining correction data, which reflects upon the density to be realized by the chromatic color ink, based on the result of the multivaluing operation with respect to the achromatic color ink, in order to correct the density to be realized by the chromatic color ink;
chromatic dot formation determination means for carrying out a multivaluing operation with respect to the at least two different dots having different densities per unit area, based on the corrected density to be realized by the chromatic color ink, and determining whether or not the at least two different dots are to be formed; and
head driving means for driving the head based on results of the determination by the achromatic dot formation determination means and the chromatic dot formation determination means, in order to form the at least two different dots having different densities per unit area by the chromatic color ink and the achromatic dot by the achromatic color ink.
In the third printing system according to the present invention, formation of the achromatic dot by the achromatic color affects the formation of the at least two different dots having different densities per unit area by the chromatic color. The achromatic dot is considered to include the component of the chromatic color. This structure thus appropriately controls the on/off state of the chromatic dot, based on the on/off state of the achromatic dot (for example, black dot).
The present invention is also directed to a fourth printing system with a head, which forms at least two different dots having different densities per unit area on an object, for recording a multi-tone image by a distribution of the dots, the fourth printing system including:
input means for successively receiving a tone signal of each target pixel included in an image to be printed;
first dot formation determination means for carrying out a multivaluing operation with respect to a selected dot among the at least two different dots having different densities per unit area, based on the input tone signal, and determining whether or not the selected dot is to be formed;
difference computing means for computing a difference between the input tone signal and a printing density realized by the selected dot;
second dot formation determination means for carrying out a multivaluing operation with respect to another dot among the at least two different dots having different densities, based on the difference, and determining whether or not the another dot is to be formed; and
head driving means for driving the head based on results of the determination by the first dot formation determination means and the second dot formation determination means, in order to form the at least two different dots having different densities per unit area.
The fourth printing system of the present invention first carries out a multivaluing operation with respect to a selected dot among the at least two different dots having different densities per unit area. The system then carries out a multivaluing operation with respect to another dot among the at least two different dots having different densities, based on a difference between the input tone signal and a printing density realized by the selected dot. The head is subsequently driven to form the at least two different dots having different densities per unit area.
In accordance with one preferable application of the fourth printing system, the difference computing means includes:
first effect computing means for calculating a first effect on the multivaluing operation with respect to the another dot, based on the tone signal and a recording density realized by the selected dot; and
second effect computing means for calculating a second effect on the multivaluing operation with respect to the another dot, based on the printing density realized by the selected dot,
the difference computing means computing the difference by taking into account the first effect and the second effect.
In this structure, the difference is computed by taking into account the degree of these first effect and the second effect. Varying the degree of these effects reflects upon the determination of formation or non-formation of the another dot based on the difference by the second dot formation determination means.
In accordance with another preferable application, the fourth printing system further includes error diffusion means for computing a difference between a printing density realized by the another dot based on the input tone signal and a printing density realized by the another dot as a density error, based on the determination of dot formation by the second dot formation determination means, and distributing the density error to peripheral pixels in the vicinity of a current target pixel of dot formation, in order to reflect upon the determination of dot formation with respect to the peripheral pixels by the second dot formation determination means. This structure ensures the advantages of the error diffusion technique (reduction of the mean density error and improvement in printing quality).
In the fourth printing system of the present invention, the first dot formation determination means or the second dot formation determination means may determine formation or non-formation of the dot by a dither method. When the dither method is adopted, a threshold matrix of discrete dither is preferably used.
In accordance with one preferable application, the head forms at least two different dots of different diameters as the at least two different dots having different densities per unit area. This application corresponds to a printing system with a mechanism of varying the dot diameter. Other possible structures to form the at least two different dots having different densities per unit area include a structure using at least two inks of different dye densities and a structure for changing the number of times of discharging ink of a fixed density at substantially the same place.
The head for forming the at least two different dots having different densities or different diameters may have a mechanism for discharging ink particles under a pressure applied to each ink running through an ink conduit by application of a voltage to a piezoelectric element arranged in the ink conduit. Alternatively the head may have a mechanism for discharging ink particles under a pressure applied to each ink running through an ink conduit by air bubbles that are produced by a supply of electricity to a heating body arranged in the ink conduit. These mechanisms give very fine ink particles and enable adequate regulation of the amount of each ink. A number of nozzles for spraying the ink particles may be formed on the head. In this case, a plurality of nozzles are arranged in a feeding direction of a sheet of paper, on which the multi-tone image is printed, for each color ink of each density. This structure enhances the printing rate.
The present invention is also directed to a first method of recording a multi-tone image by a distribution of at least two different dots having different densities per unit area, which are formed on an object by a head, the first method including the steps of:
successively receiving a tone signal of each target pixel included in an image to be printed;
specifying a recording density to be realized by at least a selected one of a dot having a higher density per unit area, that is, a higher-density dot, and a dot having a lower density per unit area, that is, a lower-density dot, which are both included in the at least two different dots having different densities, based on the input tone signal;
carrying out a multivaluing operation based on the specified recording density, and determining whether or not the selected one of the higher-density dot and the lower-density dot is to be formed;
making a result of the multivaluing operation with respect to the selected dot reflect upon a recording density to be realized by the other one of the higher-density dot and the lower-density dot, causing the other one of the higher-density dot and the lower-density dot to be subjected to a multivaluing operation according to the reflected recording density, and determining whether or not the other one of the higher-density dot and the lower-density dot is to be formed; and
driving the head based on results of the determination with respect to the selected dot and the other dot, in order to form the at least two different dots having different densities per unit area.
This structure enables a density error due to formation of the selected one of the higher-density dot and the lower-density dot to be reduced by formation of the other one of the higher-density dot and the lower-density dot, so that the tone of an original image is reproduced by a combination of the at least two different dots.
In accordance with one preferable application, the first method determines whether or not a first dot selected among the at least two different dots having different densities per unit area is to be formed, based on the input tone signal, prior to determination for formation of the other dots. When no formation of the first dot is determined, the first method determines whether or not a second dot having a different density per unit area from that of the first dot is to be formed. In this application, the first method then computes a difference between a printing density corresponding to the input tone signal and a printing density realized by the formed dots as a density error, based on the determination of dot formation. The density error is distributed to peripheral pixels in the vicinity of a current target pixel of dot formation, in order to reflect upon the determination of dot formation with respect to the peripheral pixels.
This structure enables a density error due to formation of the selected one of the higher-density dot and the lower-density dot to be reduced by formation of the other one of the higher-density dot and the lower-density dot, so that the tone of an original image is reproduced by a combination of the at least two different dots.
The present invention is further directed to a second method of recording a multi-tone image by a distribution of at least two different dots having different densities per unit area, which are formed on an object by a head, the second method including the steps of:
successively receiving a tone signal of each target pixel included in an image to be printed;
specifying a first dot tone value, that is, a tone value to be realized by a first dot selected among the at least two different dots having different densities per unit area, based on the input tone signal;
determining whether or not the first dot is to be formed, based on the first dot tone value;
computing a correction signal by adding quantization errors distributed from peripheral processed pixels in the vicinity of the target pixel to the input tone signal;
when no formation of the first dot is determined according to the first dot tone value, determining whether or not a second dot having a different density per unit area from that of the first dot is to be formed, based on the correction signal;
driving the head based on results of the determination with respect to the first dot and the second dot, in order to form the at least two different dots having different densities per unit area; and
computing a quantization error, which is a difference between the correction signal and a tone value realized by the formed dots, as a density error, based on the results of the determination with respect to the first dot and the second dot, and distributing and diffusing the computed density error to peripheral pixels in the vicinity of the target pixel.
The second method of the present invention successively receives a tone signal of each target pixel included in an image to be printed, and specifies a first dot tone value, that is, a tone value to be realized by a first dot selected among the at least two different dots having different densities per unit area, based on the input tone signal. It is then determined whether or not the first dot is to be formed, based on the first dot tone value. The second method then computes a correction signal by adding quantization errors distributed from peripheral processed pixels in the vicinity of the target pixel to the input tone signal, and carries out the processing, based on the correction signal. When no formation of the first dot is determined, it is then determined whether or not a second dot having a different density per unit area from that of the first dot is to be formed. The head is driven, based on results of the determination of dot formation, so as to form the at least two different dots having different densities per unit area. The second method computes a quantization error, which is a difference between the correction signal and a tone value realized by the formed dots, as a density error, based on the results of the determination of dot formation, and distributes and diffuses the computed density error to peripheral pixels in the vicinity of the target pixel.
This structure enables a density error due to formation of the first dot to be reduced by formation of the second dot having a different density per unit area from that of the first dot, so that the tone of an original image is reproduced by a combination of the at least two different dots.
The present invention also includes some other applications. The first application is a structure, in which one or related ones of the input means, the error diffusion means, the first dot formation determination means, the correction signal computing means, and the second dot formation determination means are not included in the casing of the printing system but in an apparatus for outputting images to be printed. The error diffusion method as well as the first dot formation determination means and the second dot formation determination means may be realized by discrete circuits or alternatively by the software in an arithmetic and logic circuit including a CPU. In the latter case, the apparatus for outputting images to be printed, such as a computer, carries out the processing related to generation of dots. Only a mechanism for regulating discharge of inks from the head to actually form the generated dots, for example, on a sheet of paper, is disposed in the casing of the printing system. Another possible structure divides these required means into two groups, and enables one group to be realized in the casing of the printing system and the other group to be realized in the apparatus for outputting images.
The second application is a portable recording medium, on which the software loaded to the computer system for execution is recorded. At least part of the input means and the dot formation determination means may be realized by an arithmetic and logic circuit including a CPU (hardware) and a software program executed thereon. At least part of the software program is stored on the portable recording medium.
The third application is a program supply apparatus for supplying the software program via a communications line.
The fourth application is an ink cartridge used in any one of the printing systems of the present invention. In color printing with at least two inks of different dye densities to form at least two different dots having different densities per unit area, in accordance with one preferable structure, the ink cartridge includes a black ink cartridge and a color ink cartridge that is separate from the black ink cartridge and reserves a plurality of color inks including at least two inks of different densities. This structure allows the black ink cartridge that is more frequently used for printing characters and the color ink cartridge to be replaced at arbitrary timings.
In the ink cartridge used for the printing system with the inks of different densities, at least two inks having an identical hue but different densities are arranged adjacent to each other. In accordance with a concrete structure, cyan ink, ink having a lower dye density than the cyan ink, magenta ink, ink having a lower dye density than the magenta ink, and yellow ink are arranged in this sequence in the ink cartridge.